Quadrature phase signals (I and Q) from a pulsed Doppler medical instrument are analyzed to detect phase changes of consecutive echoes from tissue at a depth of interest. Such phase changes are analyzed to determine tissue displacement. Relative displacements at different depths are examined to detect tissue expansion and contraction. The area of the uterus of a pregnant female can be scanned and the frequency of tissue displacement or expansion and contraction examined to determine pulsations due to blood supplied by the fetal heart and/or pulsations due to blood supplied by the maternal heart. Conventional M-mode or B-mode images can be colorized differently for different frequencies of tissue displacement.